THE link between climate change and human activity is now as certain as the link between smoking and cancer, experts have warned.
Ireland's economy, wildlife and coastal communities are all threatened by global warming as a major new scientific report pinned the blame on human activity.
The climate change of recent decades is unprecedented over millennia and it is now 95pc certain human activity is the main cause, according to the United Nations Intergovernmental Panel on Climate Change (IPCC).
Professor Sweeney said the rise in sea levels and increased flooding caused by melting polar icecaps would have a huge impact in coastal areas, including cities like Dublin, where 50,000 people lived less than 2.5m above sea level.
Temperatures have risen almost 1C in the last century and are set to rise by 0.5C more in the next 20 years, with rainfall also rising to cause far more frequent and intense weather events.
Without strong action to limit emissions, temperatures could rise by 4C by the end of the century – a point at which "all bets were off" in terms of how global weather systems and oceans react, he said.
The IPCC report warns that oceans have warmed, snow and ice have melted, sea levels have risen and greenhouse gas concentrations are at their highest levels in 800,000 years.
The last 30 years have been the warmest of the past 1,400 years in the northern hemisphere. Sea levels are predicted to rise by up to a metre by the end of this century.
The Stop Climate Chaos coalition of charities and environmental groups warned that the economic and humanitarian impact of unchecked climate change would be "disastrous" for Ireland and the world.
Spokesman Oisin Coghlan said the Government's proposed climate change bill was "like a car without an engine" because it contained no 2050 emissions targets, and said that the final version due shortly would have to be greatly strengthened.
"It's not about our children's children, it's the children born today who will bear the brunt of this," he said.
An Taisce accused the Government of being indifferent to stopping climate change, noting the Food Harvest 2020 target of massively increasing Ireland's beef and milk output paid no heed to the impact on greenhouse gas levels this would have.
Professor Sweeney warned that this food output expansion posed a major challenge for climate change.
"Agriculture is undoubtedly the number one problem in terms of addressing Irish national emissions," he said.
No sector could be made a special case, as everyone would have to play their part in cutting gas emissions, he said.
Green Party leader Eamon Ryan said that governments and individuals everywhere needed to protect themselves from the worst ravages of climate change.
"In Ireland we can take a massive step forward by switching off our peat and coal-fired stations, and turning on to a more competitive renewals alternative," he said.
Environment Minister Phil Hogan said the report's findings were "clear and robust", and a "serious wake-up call on the need for a comprehensive global response to climate change".
The final Heads of the Climate Action and Low-Carbon Development Bill will be published early next year.
Energy Minister Pat Rabbitte said the IPCC report meant "the naysayers have had their day. I don't think that there are any deniers, or any credible deniers of climate change, still with us".
He said Ireland was doing its bit to combat it. "We are committed to producing 40pc of our energy from renewable resources by 2020 and we're going to meet that target, predominantly from wind and that's a big contribution in terms of this country."